1. Field of the Invention
The present invention relates to voltage controlled oscillators (VCO's ) and more particularly to voltage controlled oscillators incorporating complementary metal oxide silicon field effect transistor (CMOS) circuitry. Voltage controlled oscillators are widely used in phase-locked loop (PLL) circuits.
A voltage controlled oscillator operates to provide an output frequency which is linearly related to a control input voltage. Typically, this is achieved by providing a capacitor which is charged and discharged at a rate proportional to the control voltage. A comparator is provided to switch between charging and discharging when the voltage on the capacitor goes above or below first and second reference voltages, respectively.
2. Description of the Prior Art
Voltage controlled oscillators including current source and current sinks for charging and discharging a capacitor are shown in U.S. Pat. Nos. 3,886,408 to Takahashi, 3,904,988 to Hsiao, 4,236,567 to Astle and 4,321,561 to Payne et al. In both the Payne and Hsiao patents, current switching is provided by switching transistors which are located in the output current path. Although generally acceptable, such circuits are limited in that the inclusion of switching in the output current path introduces parasitic resistance and capacitance, thus slowing the switching time. In order for the VCO to operate properly, it is desirable to have the current sink or source switched as rapidly as possible so that a uniform charging rate is achieved for the capacitor. It is not possible to achieve such rapid switching with prior art circuits which include switching in the output current path.